Method And Device For Producing Hot Metallic Strip, In Particular From Lightweight Structural Steel

ABSTRACT

The object of the invention is to improve the quality of cast hot strips. The object is achieved in terms of the method by a method for producing hot metallic strips, in particular of lightweight structural steel, wherein a melt is charged in the presence of inert gas by means of a runner onto a circulating casting belt of a horizontal strip casting facility, solidifies to form a pre-strip with a thickness of between 6 and 20 mm and, after thorough solidification, the pre-strip undergoes a hot rolling process. The invention is characterized in that the heat transfer and the contact (surface area, time) between the strand solidified to form a pre-strip and the casting belt is reduced, and by a device for carrying out the method.

The invention relates to a method of producing hot metallic strips, in particular of lightweight structural steel, according to the preamble of claim 1, and to a device according to the preamble of claim 7.

A device of a type involved here for producing hot metallic strips of lightweight structural steel is known (steel research 74 (2003), No. 11/12, page 724-731).

Melt is fed in the known method from a feed vessel via a runner onto a circulating casting belt of a horizontal strip casting facility. The fed melt solidifies when undergoing intense cooling to form a pre-strip with a thickness in the range between 6-20 mm. After thorough solidification, the pre-strip undergoes a hot rolling process.

During solidification, material stress causes warpage of the pre-strip, adversely affecting the quality of the hot strip. In particular, some steels experience on their strand underside irregular and large-area contractions as a result of rapid cooling.

Furthermore, there is the possibility of an excessive friction between the casting belt and the solidifying strand, causing an excessive deviation in the synchronous speed between casting belt and rolling speed, so that the strand tears off in the worst case scenario.

This problem of adjustment of the synchronous speeds is always relevant when in-line casting and rolling is involved.

It is an object of the invention to provide a method and a device for producing hot metallic strips, in particular of lightweight steel, obviating the aforestated problems.

Based on the preamble, this object is solved in combination with the characterizing features of claim 1. Advantageous improvements as well as an apparatus for producing hot strips are the subject matter of the other claims.

According to the teaching of the invention, the heat transfer as well as the contact (surface area, time) between the strand solidified to a pre-strip and the casting belt is reduced. There are various ways to achieve this, with each single measure being effective by itself or also in combination.

The method according to the invention is basically suitable for the production of hot strips of various metallic materials, in particular also for lightweight structural steel.

A first proposal aims to reduce the contact time between casting belt and solidifying strand. This is realized by causing the casting belt to locally vibrate with the aid of an electromagnetic system. This involves the arrangement of an electromagnetic system which function like a loudspeaker, below the casting belt. It is crucial for proper operation to install the system at a site where a solid strand shell has already been formed.

A further proposal is directed to the reduction of the heat transfer. This involves charging a gas, in particular a mixed gas of inert and reducing gas, in the charging zone of the melt between runner and casting belt. The reducing gas is preferably hydrogen.

Gas acts advantageously across the entire width of the casting belt. The gas volume flow is slight and resembles more a blanketing. When the volume flow is excessive, the planar formation of the strand's underside would be adversely affected. The applied mixed gas provides for the strand to have an underside surface which is substantially scale-free. A blank surface means less heat radiation so that the heat transfer is significantly reduced between the solidifying band and the casting belt.

A third proposal involves a structuring of the casting belt and has also proven to be very effective. Longitudinal embossments are advantageously impressed in the casting direction. As an alternative, nubs may be arranged in spaced-apart relationship across the casting belt. The application of longitudinal embossments has the advantage of a fairly simple production by drawing a smooth band through the profiled pair of rolls.

It is ensured that any kind of structuring of the casting belt leads to a decrease of the heat transfer between the solidifying strand and the casting belt. The reproduction of the embossments by the melt causes, as a result of shrinkage during solidification, a local detachment of the casting shell and accompanying reduction of the contact surface. This means a decrease in the heat transfer and friction between stand and casting belt and this can be exploited to enhance the process reliability in in-line manufacturing of casting and rolling.

The casting speed should ideally be in synchronism with the rolling speed in in-line manufacturing. In reality however, there are oftentimes deviations which must not be excessive as the pre-strip would otherwise tear off. Deviations of the synchronous speed of >0.5 m/s for example are considered problematic. If such deviations cannot be controlled, a buffer, also called looper, must be installed anteriorly of the roll stand.

The method according to the invention will now be described in greater detail with reference to a drawing, in which:

FIG. 1 a shows a frontal view of the structure of the casting belt in accordance with the invention,

FIG. 1 b a cross section in the direction A-A in FIG. 1 a,

FIG. 2 shows a length section of the arrangement according to the invention of an electromagnetic system,

FIG. 3 shows a length section of the blanketing of the strand's underside,

FIG. 4 shows a top view of FIG. 3.

FIG. 1 a shows a frontal view of a structure of the casting belt 1 in accordance with the invention. The trailing deflection pulley 2, as viewed in transport direction, can be seen as well as the casting belt 1 placed thereupon and advancing in arrow direction 3. Illustrated are the conjointly moving side boundaries 4, 4′ on the top side.

Rolled into the casting belt 1 are embossments 5 arranged in length direction. The cutaway illustration FIG. 1 b provides easy depiction of a detail of the cross section in direction A-A in FIG. 1 a.

FIG. 2 shows a second proposal for reducing the heat transfer, with the illustration depicting a length section of the charging zone of a horizontal strip casting facility. The facility includes as main element a melting vessel 6, an inlet 7 with attached runner 8. Melt 9 contained in the melting vessel 6 exits the runner 8 and is fed onto a circulating casting belt 1.

To excite local vibrations of the casting belt 1, an electromagnetic system 10 is arranged beneath the casting belt 1. It operates in accordance with the principle of a loudspeaker and causes the casting belt 1 to vibrate. This results in a shortening of the contact times of the solidifying melt with the casting belt 1. The excitation of vibrations is possible only when the melt bath has formed a sufficiently solid casting shell on the underside. The electromagnetic system 10 must therefore be arranged further away from the charging zone.

A third proposal for solution shows FIG. 3, in which a same length section is shown of a strip casting facility like in FIG. 2 so that same reference signs are used for same parts.

The third proposal for solution is characterized by a blanketing with a mixed gas before melt 9 is charged onto the casting belt 1. For that purpose, a hollow body 12 is arranged beneath the runner 8 and above the leading deflection pulley 11. A brush 13 is placed in front of the hollow body 12 for sealing and better distribution across the width of the casting belt 1.

The hollow body 12 is connected to a feed conduit 14 (FIG. 4) for supply of gas. After starting the gas supply, the mixed gas exits the hollow body 12 and flows along the gap between casting belt 1 and underside runner 8 directly to the charging zone. As a result, the casting shell that initially solidifies is prevented from scaling. It remains substantially blank.

List of Reference Signs No. Designation 1 Casting belt 2 Trailing deflection pulley 3 Rotation direction 4, 4′ Side boundary 5 Longitudinal embossment 6 Melting vessel 7 Inlet 8 Runner 9 Melt 10 Electromagnetic system 11 Leading deflection pulley 12 Hollow body 13 Brush 14 Feed conduit 

1.-16. (canceled)
 17. A method of producing a hot metallic strip, comprising the steps of: charging a melt by means of a runner onto a circulating casting belt of a horizontal strip casting facility in the presence of inert gas; allowing the melt to solidify to form a pre-strip with a thickness between 6 to 20 mm; and causing the casting belt to locally vibrate so as to reduce a heat transfer and a contact between the solidifying melt and the casting belt.
 18. The method of claim 17, further comprising the step of subjecting the pre-strip to a hot rolling process after solidification.
 19. The method of claim 17, wherein the melt is made of lightweight structural steel.
 20. The method of claim 17, wherein the step of reducing the heat transfer includes the step of decreasing a contact area between the solidifying melt and the casting belt.
 21. The method of claim 17, wherein the step of reducing the heat transfer includes the step of decreasing a contact time between the solidifying melt and the casting belt.
 22. The method of claim 17, wherein the casting belt is excited electromagnetically.
 23. The method of claim 17, wherein the step of reducing the heat transfer includes the step of feeding a gas between the runner and the casting belt before the charging step.
 24. The method of claim 23, wherein the gas is a mixed gas of an inert gas as carrier and a reducing gas.
 25. The method of claim 24, wherein the reducing gas is hydrogen.
 26. A device for producing a hot metallic strip, comprising: a feed vessel which contains the melt and has a horizontal runner; a primary cooling zone downstream of the feed vessel and including two deflection pulleys and a circulating cooled casting belt routed over the deflection pulleys for allowing melt, received from the runner, to solidify, said casting belt being configured with a structure; a secondary cooling zone downstream of the primary cooling zone and including an enclosed roller table for further solidification of the melt to form a pre-strip; and a vibratory unit arranged beneath the casting belt to cause vibrations of the casting belt.
 27. The device of claim 26, further comprising a roll stand for subjecting the pre-strip to a rolling process.
 28. The device of claim 26, wherein the melt is made of lightweight structural steel.
 29. The device of claim 26, wherein the casting belt is defined by a longitudinal axis, said structure being realized by embossments extending in a direction of the longitudinal axis.
 30. The device of claim 26, wherein the structure is realized by providing nubs distributed across a surface of the casting belt.
 31. The device of claim 26, wherein the vibratory unit is an electromagnetically vibratory unit.
 32. The device of claim 31, wherein the electromagnetic vibratory unit arranged in a region in which the pre-strip forms a solid casting shell.
 33. The device of claim 26, further comprising a hollow body arranged in a region of a leading one of the defection pulleys beneath the runner transversely to the casting belt, said hollow body having a broad slit for connection with a gas feed line.
 34. The device of claim 33, wherein the hollow body extends across an entire width of the casting belt.
 35. The device of claim 33, further comprising a seal arranged beneath a discharge zone of the hollow body and connected with the hollow body, said seal resting upon the casting belt.
 36. The device of claim 35, wherein the seal is a brush. 